An example of conventional exhaust gas reflux apparatus is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2004-169687 (corresponding to U.S. Pat. No. 6,892,714). The disclosed exhaust gas reflux apparatus is configured such that a reflux of exhaust gas into a combustion chamber is controlled according to the opening degree of a throttle valve.
More particularly, the exhaust gas reflux apparatus shown in JP 2004-169687A includes a pair of supports disposed on a cylinder head, an auxiliary rocker shaft supported by the supports, an auxiliary rocker arm placed between the supports and pivotably and axially slidably supported by the auxiliary rocker shaft, an interlock pin protruding from an intake rocker arm and axially slidably fitted in a slot formed in one end of the auxiliary rocker arm, a gap adjustment bolt threaded onto the other end of the auxiliary rocker arm, a connection piece formed on an exhaust rocker arm correspondingly to the gap adjustment bolt, and a negative pressure actuator operable to move the auxiliary rocker arm along the auxiliary rocker shaft via a shaft fork.
When the opening degree of the throttle valve reaches a predetermined value during operation of the engine, a negative pressure acting on the negative pressure actuator exceeds a predetermined value whereupon the actuator operates to pull the shift fork to move the auxiliary rocker arm toward the exhaust rocker arm so that the gap adjustment bolt rides on the connection piece of the exhaust rocker arm. When an intake rocker arm rocks to open an intake valve during the intake stroke, the interlock pin causes the auxiliary rocker to rock in an interlocked manner to press down the connection piece via the gap adjustment bolt. As a result, the exhaust rocker arm rocks to slightly open the exhaust valve. In this way, when the exhaust valve is opened during the intake stroke, the exhaust gas remaining on the side of an exhaust port is sucked or drawn into a combustion chamber, that is, a reflux of exhaust gas occurs during the intake stroke of the engine.
In the disclosed exhaust gas reflux apparatus, the negative pressure actuator for achieving the exhaust gas reflux is operative only when the throttle valve has a predetermined middle opening degree. Furthermore, due to the use of the auxiliary rocker arm, the shift fork and the actuator, the conventional exhaust gas reflux apparatus is relatively large in size and complicated in construction, which will increase the overall size and weight of the engine.
In small-sized multipurpose engines for use in lawnmowers, for example, there is provided a governor for automatically regulating the opening degree of a throttle valve according to load variations from a start-up of the engine so that the engine speed reaches a predetermined operating speed. By virtue of the governor thus provided, the operability of the engine is considerably improved. However, in order to reduce the load on a human operator, a further reduction in size and weight of the small-sized multipurpose engines is highly desirable. As for an exhaust gas reflux mechanism to be incorporated in such small-sized multipurpose engines, consideration must be given not to increase the size and weight of the engine.
It is therefore an object of the present invention to provide an exhaust gas reflux mechanism for a multipurpose engine, which is simple in construction and small in size and weight and, hence, is able to achieve downsizing and cost-reduction of the multipurpose engine.